\documentclass{standalone}
\usepackage{tikz}

\begin{document}
\usetikzlibrary{positioning,shapes,arrows,snakes}
\definecolor{yellowish}{HTML}{FFE699}
\definecolor{grayish}{HTML}{D9D9D9} 
\tikzset{
   rect/.style={
      align=center,
      text=black,    
      minimum width=2.5cm,
      minimum height=2cm}
}
\tikzset{
   done/.style={rect, fill=grayish }
}
\tikzset{
   todo/.style={rect, fill=yellowish}
}
\tikzset{
   arrow/.style={line width=1mm}
}
\centering

\begin{tikzpicture}[font=\sffamily\large]
    \node [done]  (codering) {Kanaal\\codering};
    \node [done, right=1.5cm of codering ]    (modulatie) {Modulatie};
    \node [done, right=1.5cm of modulatie  ]  (pulse) {Pulsvorming};
    \node[rect, cloud, cloud puffs=15.7, cloud ignores aspect, minimum width=5cm, minimum height=2cm, align=center, fill=grayish, right=1.5cm of pulse] (channel) {Draadloos\\kanaal};
    \node [done] at (-4,-4)                   (matched) {Matched\\filter};
    \node [todo, right=0.5cm of matched]      (coarse) {Grove freq.-\\correctie};
    \node [todo, right=0.5cm of coarse]       (symbol) {Tijdsync};
    \node [todo, right=0.5cm of symbol]       (fine) {Fijne freq.-\\correctie};
    \node [done, right=0.5cm of fine]         (demod) {Demodulatie};
    \node [todo, right=0.5cm of demod]        (frame) {Frame-\\detectie};
    \node [done, right=0.5cm of frame]        (decodering) {Kanaal\\decodering};
    \draw [arrow, stealth-](codering.west) -- ++(-2,0) node[left]{\emph{bits}};
    \draw[arrow, -stealth] (codering.east)--  (modulatie.west);
    \draw[arrow, -stealth] (modulatie.east)-- (pulse.west);
    \draw[arrow, -stealth] (pulse.east)--     (channel.west);
    \draw[arrow, -stealth] (channel.south) -- ++(0,-0.5) -| (matched.north);
    \draw[arrow, -stealth] (matched.east) --  (coarse.west);
    \draw[arrow, -stealth] (coarse.east)--    (symbol.west);
    \draw[arrow, -stealth] (symbol.east)--    (fine.west);
    \draw[arrow, -stealth] (fine.east)--      (demod.west);
    \draw[arrow, -stealth] (demod.east) --    (frame.west);
    \draw[arrow, -stealth] (frame.east) --    (decodering.west);
    \draw[arrow, -stealth] (decodering.east) --++(1,0) node[right]{\emph{bits (hopelijk)}};
\end{tikzpicture}
\end{document}